The present invention relates to components for use in integrated circuits. More particularly, the present invention relates to an improved wideband operational amplifier suitable for providing a voltage or current feedback amplifier or a closed loop buffer.
With the growing demand for cheaper, and yet more reliable integrated circuit components for use in communication, imaging and high-quality video applications continuing to increase, integrated circuit components, such as operational amplifiers, continue to improve at an ever-rapid pace. As a result, integrated circuit manufacturers are requiring more specialized, as well as more general purpose, integrated circuit components to meet the design requirements of a myriad of emerging applications.
With respect to integrated circuits in general, a problem complicating the design of integrated circuit components, such as operational amplifiers, is the introduction of undesirable disturbances, such as noise or ringing, from one part of the circuit component to another. Typically, these disturbances are imparted through various locations, including current and voltage references used the integrated circuit. With respect to operational amplifiers, these disturbances are generally imparted within the input and output stages, as well as the power supply rails.
For integrated circuits, many recent current sources have incorporated degeneration resistors between the supply rails and the current source""s components, such as a current mirror""s transistors, to provide the current source with a higher output resistance. Unfortunately, for a given voltage at the input or output terminals of an integrated circuit component, the increasing in value of the degeneration resistors tends to cause the circuit""s components, such as the transistors in a current mirror, to saturate and thus cause the current source to eventually fail at higher currents. Further, these current sources, in general, can be configured to either sink or source current as needed, but have great difficulty, or can not provide altogether, both the sourcing and sinking of current as needed to substantially absorb external disturbances imparted onto the current source. Accordingly, these current sources tend to introduce the remaining noise and disturbances to the remaining components of the integrated circuit, such as an operational amplifier.
The demands for improvement in operational amplifiers exist in many areas, including operational amplifiers having lower input offset voltage, higher slew rates, higher voltage and current output while requiring lower supply current, lower input noise, and greater stability with regard to external disturbances such as ringing. While many recent operational amplifiers have been developed to provide a slew-boosted input stage, in general, these operational amplifiers tend to have a poorer common-mode input voltage range, lower output voltages, and/or lower output current at higher output voltage. Other recent operational amplifiers have provided a boosted output stage capable of delivering high output currents using lower supply currents, unfortunately, however, these operational amplifiers tend to suffer from deplorable crossover distortions, e.g., unacceptable 3rd harmonic distortions within the output stages.
An additional problem existing with operational amplifiers is input offset voltage. Input offset is generally the magnitude of the voltage that if applied to the input(s) of an operational amplifier would reduce to zero the output voltage of the operational amplifier. Typically, this offset voltage is a result of mismatches and internal biases, such as, for example, unequal PNP and NPN betas or impedance values, existing within the various components, e.g., transistors, capacitors and resistors, that comprise the operational amplifier. Accordingly, an input offset voltage can cause various problems in the application of the amplifier.
Further, with respect to operational amplifiers, many recent output stage circuits have began incorporating current feedback amplifiers configured as buffers in an attempt to provide an alternative to the use of conventional emitter/source followers. In general, the feedback resistor employed in these current feedback buffers is often configured to set the phase margin for the output stage circuit, i.e., determine the instability in the output circuit. Typically, if an amplifier possesses a phase margin of less than 180 degrees, the amplifier is stable. If on the other hand, the phase margin of an amplifier exceeds 180 degrees, the amplifier will tend to be unstable. By increasing the value of the feedback resistor, the phase margin of the output circuit can be improved, however, this increasing of the feedback resistor value has the disadvantage of reducing the bandwidth of the output stage circuit. Further, by reducing the bandwidth of the output stage circuit, the phase margin for the rest of the operational amplifier may be adversely affected.
Other attempts to improve the phase margin, and thus the stability, of an output stage have demonstrated some success, but disadvantages still exist with these newer implementations. For example, some operational amplifiers employ capacitors between the input node of an output stage and the supply rails. Unfortunately, due to parasitic inductances typically occurring in the supply rails, VCC and VEE, multiple feedback paths are created in the output stage, thus potentially leading to marginal stability and severe ringing.
Accordingly, as one will appreciate, a need exist for improved integrated circuit components capable of reducing the detrimental effects of noise introduced by external and internal components within an integrated circuit. Further, a need exist for an improved operational amplifier having a fast slew rate and configured to provide a high voltage and current output while solving the problem of package parasitics and multiple feedback paths within the amplifiers.
The integrated circuit components according to the present invention addresses many of the shortcomings of the prior art. In accordance with a preferred embodiment of the present invention, a wideband operational amplifier configured to provide a high output voltage and current comprises an input stage having a first input buffer, a second input buffer and a bias rail buffer, and an output stage having an output buffer and a compensation circuit. Further, the first and second input buffers may include current mirrors suitably configured to facilitate a lower input offset voltage and lower input voltage noise. Additionally, the operational amplifier may provide wide common-mode input range and full power bandwidth simultaneously.
In accordance with another aspect of the present invention, a bias rail buffer comprising a buffer portion and a current source is suitably configured to provide a stable bias rail reference by sourcing and sinking current thus substantially eliminating the introduction of ringing or other external disturbances from one part of an integrated circuit component, such as an operational amplifier, to another.
In accordance with yet another aspect of the present invention, current mirrors configured to provide current sources to an integrated circuit component suitably provide more degeneration for lower noise and higher output resistance without increasing the minimum input and output voltages of the current mirrors.
In accordance with a further aspect of the present invention, a compensation circuit is configured to provide compensation for multiple feedback paths introduced by parasitic inductances, capacitances or resistances existing in the supply rails into an output stage buffer.